The present invention relates to a reliable, low-cost process for producing rheocast ingots of light alloy, particularly aluminium alloy to which ceramic particles are added, and from which to die cast internal combustion engine components.
U.S. Pat. No. 4,310,352, entitled: "Process and device for preparing a metal alloy mixture comprising a solid phase and a liquid phase," and the content of which is incorporated herein as required purely by way of reference, relates to a static mixer consisting of a cylindrical runner housing a succession of helical blades, and enabling a metal alloy to be poured and partially solidified as it flows through the mixer, while at the same time mixing the solid phase so formed with the remaining liquid phase, to produce, at the outlet of the mixer, a relatively low-viscosity solid/liquid mixture in which the segregated solid phase is uniformly suspended in the liquid alloy.
The mixture so formed remains stable long enough for it to be ladled and cast. To achieve the above characteristics, the solid/liquid mixture must be produced under stationary fluid-dynamic conditions, and provision must be made for accurately and rapidly controlling the physical and dynamic parameters involved (temperature, alloy cooling gradient, speed through the static mixer, etc.). For this purpose, the Applicant has devised a perfected semiliquid casting process as described in U.S. Pat. No. 5,119,977, entitled: "Continuous semiliquid casting process and furnace," and the content of which is incorporated herein as required purely by way of reference. According to the above process, the static mixer is connected to a pressurized tilt furnace for enabling casting under stationary conditions.
Metal alloys cast using the above semiliquid processes are known as "rheocastings", and present particularly good microstructural characteristics. In particular, rheocast light alloy has recently been found to present a globular as opposed to the usual denditric structure, thus providing for improved mechanical characteristics and workability. Semiliquid casting processes, however, cannot be employed as such for producing internal combustion engine components, which, for reasons of economy and the complex design of the components, are die cast, an operation which, by virtue of the high injection speeds involved, is performed under turbulent flow conditions. Moreover, die casting does not permit the use of several recent high-performance metal alloys incorporating a predetermined percentage of ceramic particles or fibers in the matrix.
To overcome the above drawbacks, the Applicant has devised a semiliquid die casting process employing rheocast ingots of light alloy, with or without ceramic particles, as described in U.S. patent application Ser. No. 07/870,494, now abandoned, entitled: "Process for producing high-mechanical-performance die castings via injection of a semiliquid metal alloy," and the content of which is incorporated herein as required by way of reference.
Despite presenting excellent structural characteristics, a drawback of die castings produced using the above process is that they do not allow of heat treatment. This is due to the die casting ingots having to be formed of the same weight as the component being produced, for which purpose, according to the above process, they are cut from a rheocast ingot produced by casting the semiliquid alloy from the static mixer (with or without ceramic particles) inside an ingot mold. Unfortunately, in the course of the above operation, turbulent flow is originated inside the ingot mold, thus resulting in gaseous substances being incorporated in the alloy and subsequently in the die castings, and which, during heat treatment, may possibly result in damage to the die castings or, at least, a poor surface finish (so-called "orange peel" effect) incompatible with applications requiring a good surface finish.
Semiliquid rheocasting in ingot molds also presents numerous additional drawbacks. Firstly, the ingot molds must be perfectly dry, in that, particularly in the case of aluminium alloys, even the slightest amount of humidity results in uncontrolled spatter seriously endangering the safety of the operators. Secondly, for enabling extraction of the rheocast ingots from the mold, this must be sharply cone-shaped, so that the resulting ingot presents a variable section along the longitudinal axis, thus complicating automatic cut-off of the die casting ingots, which, for a given length, differ in weight depending on the axial position in which the rheocast ingot is cut. Thirdly, any change in the crosswise dimension of the die casting ingots (e.g. for switching from one production component to another) entails changing the ingot molds. And finally, due to shrinkage during solidification, part of the rheocast material in the ingot mold is unusable and therefore scrapped.